18 December - CCTV coverage casts doubt on police officers' version of events

19 December - 23-year-old man arrested

Mr Cameron, speaking on a trip to Afghanistan, praised the former chief whip's "calm and rational" response to the affair, adding that he had "a lot of sympathy" for his predicament.

Two people have been arrested during Operation Alice - a policeman and a 23-year-old man not employed by the police.

The BBC understands that Mr Mitchell has met Pat Gallan, a deputy assistant commissioner, who is in charge of the Metropolitan Police's Directorate of Professional Standards and is overseeing the investigation.

Political correspondent Gary O'Donoghue said he had been told Mr Mitchell regarded Ms Gallan as "extremely good" though he no longer had "confidence" in the Met commissioner himself.

Earlier this week, Mr Hogan-Howe publicly backed the two original officers who were present during the altercation in Downing Street.

There has also been what is described as an "unsatisfactory" exchange of letters between Mr Mitchell and the commissioner, which has reportedly further eroded his confidence.